The use of ion exchangers, both organic and inorganic, including crystalline molecular sieve zeolites, in order to remove certain metals from aqueous solutions is notoriously old in the art and the patent and technical literature contains many examples of such techniques. Although molecular sieves generally are effective for the removal of certain cations, nevertheless, when competing cations are present in the aqueous solution, a molecular sieve will function normally to the point at which the metal which is desirous of being removed effectively occupies some portion of the ionic sites in said zeolite. Thereafter, the zeolite must either be discarded or regenerated.
A very practical use for the above type of operation is in the home water softening industry wherein an ion exchanger of the organic or inorganic type is contacted with water until the calcium and magnesium ions which are inherently present in most mineral water replaces the ion originally associated with the ion exchanger, usually sodium. At this point, the ion exchanger has to be regenerated and this is usually accomplished by back-washing, or back-flushing, or otherwise contacting the ion exchanger with a solution of a different cation than that which was removed from the water, i.e., usually sodium in the form of sodium chloride. The sodium exchanges for the calcium/magnesium in the spent ion exchanger and the cycle is ready to start anew.
In evaluating the properties of a suitable ion exchanger, it is quite obvious that the environment in which it works to remove the unwanted metal or metals is of extreme importance and its susceptibility to competing ions is of paramount importance in obtaining a practical exchanger as opposed to one that is merely a scientific curiosity.
Thus, for example, in industrial processes wherein heavy metals are present in contaminated aqueous solutions, such heavy metals are not ordinarily present by themselves because the water contains other ions, particularly calcium and magnesium. Thus for an ion exchanger to be practical in the contact of industrial waste streams containing heavy metals, it is necessary that the ion exchanger be sufficiently selective towards heavy metals versus magnesium or calcium which compete for its ion exchange sites.
U.S. Pat. No. 5,053,139 discloses that certain amorphous titanium and tin silicate gels demonstrate remarkable rates of uptake for heavy metal species such as lead, cadmium, zinc, chromium and mercury which are an order of magnitude greater than that of prior art absorbents or ion exchangers under the conditions tested which include the presence of competing ions such as calcium and magnesium. The combination of extraordinary lead selectivities, capacity and uptake rates, allows such materials to strip lead from aqueous streams with minimal contact time allowing direct end use in filters for water purification, be it under-the-counter or under-the-faucet, or whole-house devices. While this patent teaches a process for the removal of heavy metals from aqueous solutions containing competing ions such as calcium and/or magnesium using an amorphous titanium or tin silicate, no information is provided for the selective removal of Group I or II ions, such as cesium or strontium from aqueous streams containing competing ions.
Throughout the nuclear industry, many aqueous streams exist containing radioactive ions such as strontium and cesium which must be removed prior to disposal of the liquid. Ion exchange is an ideal methodology to remove such ions. However, these streams generally contain non-radioactive competing cations that render most ion exchange materials ineffective due to limited selectivity. There are many different streams containing various levels of different competing ions. For example, the Fukushima, Japan site is known to have large quantities of water containing radioactive strontium and cesium, complicated by contamination with substantial levels of seawater due to the tsunami of 2011. Removing the radionuclides in this competing ion environment has been challenging.
Another example of high competing ions is found in high level nuclear waste solutions. These solutions, proposed materials and test methods are reviewed by Hobbs, D. T., et al in “Strontium and Actinide Separations from High Level Nuclear Waste Solutions Using Monosodium Titanate 1. Simulant Testing”, Separation Science and Technology, 40: 3093-3111, 2005. Hobbs discloses that monosodium titanate (MST), NaTi2O5.xH2O, an amorphous white solid, exhibits high selectivity for many metallic ions in both acidic and alkaline waste solutions including those containing strontium and several actinides. To those skilled in the art, it is well know that very expensive and specialized mono sodium titanates (MST) and crystalline silicotitanates (CST) are employed for the purification of these streams.